The present invention relates to a method of manufacturing an ethylene-vinyl acetate copolymer, and more particularly to a method of manufacturing an ethylene-vinyl acetate copolymer which can suppress formation of gelled substance and scales, and which can produce the copolymer as a raw material of saponified products of ethylene-vinyl acetate copolymer which have excellent moldability.
Since saponified products of ethylene-vinyl acetate copolymer are superior in various properties such as gas barrier properties and mechanical strength, they are widely used for various applications as films, sheets and fibers.
Such a saponified product is manufactured by copolymerizing ethylene and vinyl acetate and then saponifying the copolymer obtained. Thus an ethylene-vinyl acetate copolymer is also an important raw material for the saponified product.
For continuous polymerization of an ethylene-vinyl acetate copolymer, a high-pressure polymerizing vessel with a jacket, coil or a reflux condenser is normally used into which ethylene and vinyl acetate, which are raw materials, are introduced together with an alcoholic solvent such as methanol. But it has been a problem that the formation of gelled products or the growth of scales on the wall surface of the polymerizing vessel may hinder a long run of continuous polymerization.
As a countermeasure against this problem, JP patent publication 2-52922 discloses a method in which vinyl acetate (and a solvent) is passed through a heat exchanger provided outside a polymerizing vessel to cool it, letting the ethylene taken out of the polymerizing vessel absorbed into the vinyl acetate and solvent, and introducing the mixture into the polymerizing vessel to vaporize the ethylene in the vessel, thereby removing the polymerizing heat.
The present applicant studied such a method and found out that in order to effectively carry out such a method, it is necessary to considerably cool the vinyl acetate and solvent to be introduced into the polymerizing vessel and increase the amount of the ethylene to be absorbed, but if the amount of ethylene is excessive, foaming occurs in the polymerizing vessel, causing pollution of the polymerizing vessel or clogging of the piping.
Therefore, as measures for preventing the formation of scale in polymerization using a conventional polymerizing vessel provided with a jacket or coil for cooling, the present applicant proposed a method of cooling the top plate portion of the polymerizing vessel (JP patent publications 11-116637, 11-116638).
But it was found out that in the abovesaid method, although it can prevent the formation of scale on a vapor phase portion which does not contact the polymerizing reaction liquid, the effect of preventing scales at the liquid-contact portion is insufficient.
According to the present invention, there is provided a method of manufacturing an ethylene-vinyl acetate copolymer wherein continuous polymerization of ethylene and vinyl acetate is carried out in a polymerizing vessel provided with a jacket and/or a coil for cooling on condition that the heat transfer area of the jacket and/or coil and the polymerizing calorific value per hour satisfy the following formula:
A less than Q/2000
wherein A is the heat transfer area (m2) and Q is the calorific value (kcal/hr) for polymerization.